Multilayer structure based on polyamides and on a tie layer made of a copolyamide blend

ABSTRACT

A multilayer structure based on polyamides, comprising:  
     a first layer ( 1 ) formed from a polyamide P 1  or from a blend of a polyamide P 1  and a polyolefin PO 1  having a P 1  polyamide matrix,  
     optionally, a layer ( 2   a ) formed from EVOH;  
     a layer ( 2 ) formed from a blend of PA-6/12 copolyamides, one comprising by weight more 6 than 12 and the other more 12 than 6;  
     a layer ( 3 ) formed from a polyamide P 3 ,  
     it being possible for P 1  and P 3  to be identical or different, the layers ( 1 ), ( 2 ), ( 2   a ) and ( 3 ) being successive and adhering to one another in their respective contact regions.

[0001] This application claims the benefit of the filing date of U.S.Provisional Application Serial No. 60/358,388, filed Feb. 22, 2002.

FIELD OF THE INVENTION

[0002] The present invention relates to structures based on polyamidesand on a tie layer made of a copolyamide blend. They comprise apolyamide layer, a layer made of a copolyamide blend and anotherpolyamide layer. It is particularly useful when one of the polyamidelayers is made of PA-12 and the other polyamide layer is made of PA-6 orbased on PA-6. These structures may include other layers, for example alayer of EVOH (an ethylene/vinyl alcohol copolymer). These structuresare useful for making tanks, containers, bottles, multilayer films,tubes and pipes. They may be manufactured by blow coextrusion. Theadvantage of these structures is that they are a barrier to manysubstances. One particularly useful use relates to tubes fortransporting petrol and in particular for taking petrol from the tank tothe engine of a motor vehicle.

[0003] For safety and environmental protection reasons, motor-vehiclemanufacturers require these tubes to have both mechanical propertiessuch as strength and flexibility with good cold (−40° C.) impactstrength as well as good high-temperature (125° C.) strength, and alsovery low permeability to hydrocarbons and to their additives,particularly alcohols such as methanol and ethanol. These tubes mustalso have good resistance to the fuels and lubrication oils for theengine. These tubes are manufactured by coextruding the various layersusing standard techniques for thermoplastics.

PRIOR ART AND TECHNICAL PROBLEM

[0004] Among the characteristics of the specification for these tubes,five are particularly difficult to obtain jointly in a simple manner:

[0005] cold (−40° C.) impact strength—the tube does not break;

[0006] fuel resistance;

[0007] high-temperature (125° C.) strength;

[0008] very low permeability to petrol;

[0009] good dimensional stability of the tube in use with the petrol.

[0010] In multilayer tubes of various structures, the cold impactstrength remains unpredictable before having carried out thestandardized tests for cold impact strength.

[0011] Moreover, it is already known from Patent Application EP 0 781799 that in motor vehicles, owing to the effect of the injection pump,the petrol flows at high speed in the pipes connecting the engine to thetank. In certain cases, friction between the petrol and the internalwall of the tube can generate electrostatic charges, the build-up ofwhich may result in an electrical discharge (a spark) capable ofigniting the petrol with catastrophic consequences (an explosion). It istherefore necessary to limit the surface resistivity of the internalface of the tube to a value of generally less than 10⁶ ohms/square. Itis known to lower the surface resistivity of polymeric resins ormaterials by incorporating conductive and/or semiconductive materialsinto them, such as carbon black, steel fibres, carbon fibres, andparticles (fibres, platelets or spheres) metallized with gold, silver ornickel.

[0012] Among these materials, carbon black is more particularly used,for economic and processability reasons. Apart from its particularelectrically conductive properties, carbon black behaves as a fillersuch as, for example, talc, chalk or kaolin. Thus, those skilled in theart know that when the filler content increases, the viscosity of thepolymer/filler blend increases. Likewise, when the filler contentincreases, the flexural modulus of the filled polymer increases. Theseknown and predictable phenomena are explained in “Handbook of Fillersand Reinforcements for Plastics”, edited by H. S. Katz and J. V.Milewski—Van Nostrand Reinhold Company—ISBN 0-442-25372-9, see inparticular Chapter 2, Section II for fillers in general and Chapter 16,Section VI for carbon black in particular.

[0013] As regards the electrical properties of carbon black, thetechnical report “Ketjenblack EC—BLACK 94/01” by Akzo Nobel indicatesthat the resistivity of the formulation drops very suddenly when acritical carbon black content, called the percolation threshold, isreached. When the carbon black content increases further, theresistivity rapidly decreases until reaching a stable level (plateauregion). It is therefore preferred, for a given resin, to operate in theplateau region in which a metering error will have only a slight effecton the resistivity of the compound.

[0014] Polyamide- and EVOH-based tubes for transporting petrol are alsoknown from Patent Application EP 0 731 308. These tubes may have afour-layer structure comprising, respectively, a PA-12 outer layer, atie layer, which is a grafted polyolefin, an EVOH layer and an innerlayer in contact with the petrol, comprising a blend of a polyamide anda polyolefin having a polyamide matrix.

[0015] Patent EP 428 833 discloses a three-layer tube comprising,respectively, a PA-12 outer layer, a tie layer which is a graftedpolyolefin and an EVOH inner layer in contact with the petrol. PatentsEP 428 834 and EP 477 606 disclose a five-layer tube comprising,respectively, a PA-12 outer layer, a tie layer which is a graftedpolyolefin, a PA-6 layer, an EVOH layer and a PA-6 inner layer incontact with the petrol.

[0016] U.S. Pat. No. 5,038,833 discloses a three-layer tube comprising,respectively, a PA-12 outer layer, an EVOH layer and a PA-12 inner layerin contact with the petrol.

[0017] Patent EP 1 036 968 discloses a multilayer tube based onpolyamides, characterized in that it comprises, in its radial directionfrom the inside outwards:

[0018] a first layer formed from a blend of a polyamide P₁ and apolyolefin PO₁ having a P₁ polyamide matrix or else a first layer formedfrom a polyamide

[0019] optionally, an EVOH layer;

[0020] a layer formed from a copolyamide;

[0021] a layer formed from a polyamide P₃;

[0022] it being possible for P₁ and P₃ to be identical or different, thelayers being successive and adhering to one another in their respectivecontact regions.

[0023] In the description, it is stated that the copolyamide of thecopolyamide layer is advantageously a coPA-6/12, that is to say acopolymer of caprolactam and lauryllactam, the proportions by weight ofcaprolactam to lauryllactam possibly varying in the ratio of 20/80 to80/20. It is also stated that this copolyamide layer may also be a saltof hexamethylenediamine with a dicarboxylic acid having from 6 to 12carbon atoms. The term <<copolyamide>> is not correct, neverthelessPA-6,6 (hexamethylene adipamide), PA-6,9, PA-6,10 and PA-6,12(hexamethylene dodecanamide) are disclosed for example.

[0024] Patent EP 1 162 061 discloses a multilayer tube consisting of thefollowing layers, going from the inside to the outside of the tube:

[0025] a layer based on PA-6;

[0026] an EVOH layer;

[0027] a layer consisting either of a PA-6/12 copolyamide having from 55to 80% by weight of caprolactam or a PA-6,10 or PA-6,12 polyamide or ablend of PA-6 and of PA-12;

[0028] a PA-12 layer.

[0029] These two multilayer tubes have useful properties, however, ithas been discovered that, in this type of structure, if the layer lyingbetween the EVOH layer and the outer layer is a blend of PA-6/12copolyamides, one being predominantly PA-6 and the other predominantlyPA-12, then the properties are excellent. This copolyamide blend is alsovery efficient in the structure described in patent EP 1 036 968, evenwhen there is no EVOH layer.

BRIEF DESCRIPTION OF THE INVENTION

[0030] The present invention relates to a multilayer structure based onpolyamides, comprising:

[0031] a first layer (1) formed from a polyamide P₁ or else from a blendof a polyamide P₁ and a polyolefin PO₁ having a P₁ polyamide matrix,

[0032] optionally, a layer (2 a) formed from EVOH;

[0033] a layer (2) formed from a blend of PA-6/12 copolyamides, onecomprising by weight more 6 than 12 and the other more 12 than 6;

[0034] a layer (3) formed from a polyamide P₃,

[0035] it being possible for P₁ and P₃ to be identical or different, thelayers (1), (2), (2 a) and (3) being successive and adhering to oneanother in their respective contact regions.

[0036] According to a variant of the structure of the invention, thefirst layer (1) is replaced with another layer (1 a), this other layer(1 a) being formed either from a polyamide P₁a or a blend of a polyamideP₁a and a polyolefin PO₁a having a polyamide matrix and containingelectrically conducting carbon black producing a surface resistivity ofless than 10⁶ Ω/□.

[0037] According to another variant, the structure of the inventioncomprises an additional layer (1 a) placed on the layer (1) side, thelayers (1) and (1 a) adhering to each other in their respective contactregion, this other layer (1 a) being formed either from a polyamide P₁aor a blend of a polyamide P₁a and a polyolefin PO₁a having a polyamidematrix and containing electrically conducting carbon black producing asurface resistivity of less than 10⁶ Ω/□.

[0038] Advantageously, in the above structures the polyamide P₁ or P₁ais chosen from nylon-6, nylon-6,6 and nylon-12, and preferably PA-6.

[0039] Advantageously, in the above structures the polyolefin PO₁ orPO₁a is chosen from:

[0040] polyethylene;

[0041] polypropylene;

[0042] ethylene/alpha-olefin copolymers;

[0043] ethylene/alkyl (meth)acrylate copolymers;

[0044] ethylene/alkyl (meth)acrylate/maleic anhydride copolymers, themaleic anhydride being grafted or copolymerized;

[0045] ethylene/alkyl (meth)acrylate/glycidyl methacrylate copolymers,the glycidyl methacrylate being grafted or copolymerized.

[0046] Advantageously in the above structures, the polyamide P₃ ischosen from PA-11 and PA-12 and is preferably PA-12.

[0047] The structures of the invention may be in the form of tubes inwhich the layer (1) or (1 a) is on the inside and the layer (3) on theoutside. They are useful as tubes for transporting petrol. These tubesmay be manufactured by coextrusion.

[0048] The present invention also relates to the use of the copolyamideblends of the layer (2) as compatibilizers for polyamide blends,particularly blends comprising PA-12 and at least one polyamide chosenfrom PA-6 and PA-6,6. It also relates to these blends by themselves.

DETAILED DESCRIPTION OF THE INVENTION

[0049] With regard to the polyamide P₁ or P₁a matrix of the layer 1 or 1a, it is possible to use any polyamide. The term <<polyamide>> isunderstood to mean products resulting from the condensation:

[0050] of one or more amino acids, such as aminocaproic,7-aminoheptanoic, 11-aminoundecanoic and 12-amino-dodecanoic acids, orof one or more lactams, such as caprolactam, oenantholactam andlauryllactam;

[0051] of one or more salts or mixtures of diamines, such ashexamethylenediamine, dodecamethylenediamine, metaxylylenediamine,bis-p-(aminocyclohexyl)methane and trimethylhexamethylenediamine, withdiacids, such as isophthalic, terephthalic, adipic, azelaic, suberic,sebacic and dodecanedicarboxylic acids;

[0052] or mixtures of several of these monomers, which results incopolyamides.

[0053] Aliphatic diamines are α,ω-diamines containing, between the aminoterminal groups, at least 6 carbon atoms, preferably 6 to 10 carbonatoms. The carbon chain may be linear (polymethylenediamine) or branchedor even cycloaliphatic. Preferred diamines are hexamethylenediamine(HMDA), dodecamethylenediamine and decamethylenediamine.

[0054] The dicarboxylic acids may be aliphatic, cycloaliphatic oraromatic. The aliphatic dicarboxylic acids are α,ω-dicarboxylic acidshaving at least 4, preferably at least 6, carbon atoms (excluding thecarbon atoms of the carboxylic groups) in the linear or branched carbonchain. The diacids are azelaic, sebacic and 1,12-dodecanoic acids. As anillustration of such PAs, mention may be made of:

[0055] polyhexamethylene sebacamide (PA-6, 10),

[0056] polyhexamethylene dodecanediamide (PA-6, 12),

[0057] poly(undecanoamide) (PA-11),

[0058] poly(lauryllactam) (2-azacyclotridecanone) (PA-12),

[0059] polydodecamethylene dodecanediamide-(PA-12, 12),

[0060] polycapronamide (PA-6),

[0061] polyhexamethylene adipamide (PA-6,6).

[0062] The PAs have a number-average molecular mass {overscore (M)}_(n)generally greater than or equal to 5000. Their inherent viscosity(measured at 20° C.) for a 0.5 g sample in 100 g of meta-cresol) is ingeneral greater than 0.7.

[0063] It is possible to use polyamide blends. Advantageously, PA-6 andPA-6,6 and PA-12 are used.

[0064] With regard to the polyolefins of layer (1) or (1 a), polyolefinsare understood to mean polymers comprising olefin units such as, forexample, the units: ethylene, propylene, 1-butene and their higherhomologues.

[0065] By way of example, mention may be made of:

[0066] polyethylene, polypropylene, copolymers of ethylene withalpha-olefins. These products may be grafted with unsaturated carboxylicacid anhydrides such as maleic anhydride or unsaturated epoxides such asglycidyl methacrylate;

[0067] copolymers of ethylene with at least one product chosen from (i)unsaturated carboxylic acids, their salts and their esters, (ii) vinylesters of saturated carboxylic acids, (iii) unsaturated dicarboxylicacids, their salts, their esters, their half-esters and their anhydridesand (iv) unsaturated expoxides. These ethylene copolymers may be graftedwith unsaturated dicarboxylic acid anhydrides or unsaturated epoxides;

[0068] styrene/ethylene-butylene/styrene block copolymers (SEBS), thesepossibly being maleicized.

[0069] It is possible to use blends of two or more of these polyolefins.

[0070] Advantageously, the following are used:

[0071] polyethylene;

[0072] polypropylene;

[0073] copolymers of ethylene with an alpha-olefin;

[0074] ethylene/alkyl (meth)acrylate copolymers,

[0075] ethylene/alkyl (meth)acrylate/maleic anhydride copolymers, themaleic anhydride being grafted or copolymerized;

[0076] ethylene/alkyl (meth) acrylate/glycidyl methacrylate copolymers,the glycidyl methacrylate being grafted or copolymerized.

[0077] It is recommended, in order to facilitate the formation of thepolyamide matrix, and if the polyolefins have few or no functionalgroups able to facilitate the compatibilization, to add acompatibilizer.

[0078] The compatibilizer is a product known per se for compatibilizingpolyamides and polyolefins.

[0079] Mention may be made, for example, of:

[0080] polyethylene, polypropylene, ethylene-propylene copolymers,ethylene-butylene copolymers, all these products being grafted withmaleic anhydride or with glycidyl methacrylate;

[0081] ethylene/alkyl (meth)acrylate/maleic anhydride copolymers, themaleic anhydride being grafted or copolymerized;

[0082] ethylene/vinyl acetate/maleic anhydride copolymers, the maleicanhydride being grafted or copolymerized;

[0083] the above two copolymers in which the maleic anhydride isreplaced with glycidyl methacrylate;

[0084] ethylene/(meth)acrylic acid copolymers, and possibly their salts;

[0085] polyethylene, polypropylene or ethylene/propylene copolymers,these polymers being grafted with a product having a reactive site withamines; these grafted copolymers then being condensed with polyamides orpolyamide oligomers having a single amine end group.

[0086] These products are described in Patents FR 2 291 225 and EP 342066, the contents of which are incorporated by reference in the presentapplication.

[0087] The amount of polyamide forming the matrix in the inner layer maybe between 50 and 95 parts per 5 to 50 parts of polyolefins.

[0088] The amount of compatibilizer is the amount sufficient for thepolyolefin to be dispersed in the form of nodules in the polyamidematrix. It may represent up to 20% of the weight of the polyolefin.These polymers of the inner layer are manufactured by blending thepolyamide, the polyolefin and possibly the compatibilizer using standardtechniques for melt blending (twin-screw, Buss, single-screw extruders).

[0089] These polyamide/polyolefin blends of layer 1 or 1 a may beplasticized and possibly contain fillers such as carbon black, whichallows this layer to be made antistatic or electrically conductive.

[0090] According to the advantageous embodiment of the invention, theamount of polyamide of layer 1 or 1 a is between 50 and 75 parts per 100parts of the polyamide/polyolefin blend.

[0091] Preferred embodiments of the polyamide/polyolefin blends will nowbe described. These blends may be used in the inner layer, optionallywith conducting black and/or used without conducting black.

[0092] According to a first preferred embodiment of the invention, thepolyolefin comprises (i) a high-density polyethylene (HDPE) and (ii) ablend of a polyethylene (C1) and a polymer (C2) chosen from elastomers,very low-density polyethylenes and ethylene copolymers, the (C1)+(C2)blend being cografted with an unsaturated carboxylic acid.

[0093] According to a second preferred embodiment of the invention, thepolyolefin comprises (i) polypropylene and (ii) a polyolefin whichresults from the reaction of a polyamide (C4) with a copolymer (C3)comprising propylene and an unsaturated monomer X, which is grafted orcopolymerized.

[0094] According to a third preferred embodiment of the invention, thepolyolefin comprises (i) a polyethylene of the LLDPE, VLDPE ormetallocene type and (ii) an ethylene/alkyl (meth)acrylate/maleicanhydride copolymer.

[0095] With regard to the first embodiment, the proportions (by weight)are advantageously the following:

[0096] 60 to 70% of polyamide,

[0097] 5 to 15% of the cografted blend of (C1) and (C2),

[0098] the balance being high-density polyethylene.

[0099] With regard to the high-density polyethylene, its density isadvantageously between 0.940 and 0.965 and the MFI between 0.1 and 5g/10 min. (190° C./2.16 kg).

[0100] The polyethylene (C1) may be chosen from the abovementionedpolyethylenes. Advantageously, (C1) is a high-density polyethylene(HDPE) having a density between 0.940 and 0.965. The MFI of (Cl) isbetween 0.1 and 3 g/10 min. (190° C./2.16 kg).

[0101] The copolymer (C2) may, for example, be an ethylene/propyleneelastomer (EPR) or ethylene/propylene/diene elastomer (EPDM). (C2) mayalso be a very low-density polyethylene (VLDPE) which is either anethylene homopolymer or an ethylene/alpha-olefin copolymer. (C2) mayalso be a copolymer of ethylene with at least one product chosen from(i) unsaturated carboxylic acids, their salts and their esters, (ii)vinyl esters of saturated carboxylic acids and (iii) unsaturateddicarboxylic acids, their salts, their esters, their half-esters andtheir anhydrides. Advantageously (C2) is an EPR.

[0102] Advantageously, 60 to 95 parts of (C1) per 40 to 5 parts of (C2)are used. The blend of (C1) and (C2) is grafted with an unsaturatedcarboxylic acid, that is to say (C1) and (C2) are cografted. It wouldnot be outside the scope of the invention to use a functional derivativeof this acid. Examples of unsaturated carboxylic acids are those having2 to 20 carbon atoms, such as acrylic, methacrylic, maleic, fumaric anditaconic acids. The functional derivatives of these acids comprise, forexample, anhydrides, ester derivatives, amide derivatives, imidederivatives and metal salts (such as alkali metal salts) of unsaturatedcarboxylic acids.

[0103] Unsaturated dicarboxylic acids having 4 to 10 carbon atoms andtheir functional derivatives, particularly their anhydrides, areparticularly preferred grafting monomers. These grafting monomerscomprise, for example, maleic, fumaric, itaconic, citraconic,allylsuccinic, cyclohex-4-ene-1,2-dicarboxylic,4-methyl-cyclohex-4-ene-1,2-dicarboxylic,bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic andx-methylbicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acids and maleic,itaconic, citraconic, allylsuccinic, cyclohex-4-ene-1,2-dicarboxylic,4-methylenecyclohex-4-ene-1,2-dicarboxylic,bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic andx-methyl-bicyclo[2.2.1]hept-5-ene-2,2-dicarboxylic anhydrides.Advantageously maleic anhydride is used.

[0104] Various known processes can be used to graft a grafting monomeronto the blend of (C1) and (C2). For example, this may be achieved byheating the polymers (C1) and (C2) to a high temperature, about 150° C.to about 300° C., in the presence or absence of a solvent and with orwithout a radical initiator.

[0105] In the graft-modified blend of (C1) and (C2) obtained in theabove-mentioned manner, the amount of grafting monomer may be chosenappropriately, but it is preferably from 0.01 to 10% and better stillfrom 600 ppm to 2%, with respect to the weight of grafted (C1) and (C2).The amount of grafted monomer is determined by assaying the succinicfunctional groups by FTIR spectroscopy. The MFI (190° C./2.16 kg) of thecografted (C1) and (C2) is 5 to 30 and preferably 13 to 20 g/10 min.

[0106] Advantageously, the cografted (C1)/(C2) blend is such that theMFI₁₀/MFI₂ ratio is greater than 18.5, MFI₁₀ denoting the melt flowindex at 190° C. with a load of 10 kg and MFI₂ denoting the melt flowindex with a load of 2.16 kg. Advantageously, the MFI₂₀ of the blend ofthe cografted polymers (C1) and (C2) is less than 24. MFI₂₀ denotes themelt flow index at 190° C. with a load of 21.6 kg.

[0107] With regard to the second embodiment of the invention, theproportions (by weight) are advantageously the following:

[0108] 60 to 70% of polyamide,

[0109] 20 to 30% of polypropylene,

[0110] 3 to 10% of a polyolefin which results from the reaction of apolyamide (C4) with a copolymer (C3) comprising propylene and anunsaturated monomer X, grafted or copolymerized.

[0111] The MFI (230° C./2.16 kg) of the polypropylene is advantageouslyless than 0.5 g/10 min and preferably between 0.1 and 0.5 g/10 min. Suchproducts are described in EP 647681.

[0112] The grafted product of this second embodiment of the inventionwill now be described. Firstly, (C3) is prepared, this being either acopolymer of propylene and an unsaturated monomer X, or a polypropyleneonto which an unsaturated monomer X is grafted. X is any unsaturatedmonomer that can be copolymerized with propylene or grafted onto thepolypropylene and having a functional group capable of reacting with apolyamide. This functional group may, for example, be a carboxylic acid,a dicarboxylic acid anhydride or an epoxide. As examples of monomer X,mention may be made of (meth)acrylic acid, maleic anhydride andunsaturated epoxides such as glycidyl (meth)acrylate. Advantageously,maleic anhydride is used. With regard to the grafted polypropylenes, Xmay be grafted onto propylene homopolymers or copolymers, such asethylene/propylene copolymers consisting predominantly (in moles) ofpropylene. Advantageously, (C3) is such that X is grafted. The graftingis an operation known per se.

[0113] (C4) is a polyamide or a polyamide oligomer. Polyamide oligomersare described in EP 342066 and FR 2291225. The polyamides (or oligomers)(C4) are products resulting from the condensation of the above-mentionedmonomers. Polyamide blends may be used. It is advantageous to use PA-6,PA-11, PA-12, a copolyamide having PA-6 units and PA-12 units (PA-6/12)and a copolyamide based on caprolactam, hexamethylenediamine and adipicacid (PA-6/6,6). The polyamides or oligomers (C4) may have acid, amineor monoamine terminal groups. In order for the polyamide to have amonoamine terminal group, all that is required is to use a chain stopperof formula:

[0114] in which:

[0115] R₁ is hydrogen or a linear or branched alkyl group containing upto 20 carbon atoms;

[0116] R₂ is a linear or branched, alkyl or alkenyl, group having up to20 carbon atoms, a saturated or unsaturated cycloaliphatic radical, anaromatic radical or a combination of the above. The chain stopper may,for example, be laurylamine or oleylamine.

[0117] Advantageously, (C4) is a PA-6, a PA-11 or a PA-12. Theproportion by weight of C4 in C3+C4 is advantageously between 0.1 and60%. The reaction of (C3) with (C4) preferably takes place in the meltstate. For example, it is possible to mix (C3) and (C4) in an extruderat a temperature generally between 230 and 250° C. The average residencetime of the melt in the extruder may be between 10 seconds and 3 minutesand preferably between 1 and 2 minutes.

[0118] With regard to the third embodiment, the proportions (by weight)are advantageously the following:

[0119] 60 to 70% of polyamide,

[0120] 5 to 15% of an ethylene/alkyl (meth)acrylate/maleic anhydridecopolymer,

[0121] the balance being a polyethylene of the LLDPE, VLDPE ormetallocene type; advantageously the density of this polyethylene isbetween 0.870 and 0.925, and the MFI is between 0.1 et 5 g/l min. (190°C./2.16 kg).

[0122] Advantageously, the ethylene/alkyl (meth)acrylate/maleicanhydride copolymers comprise from 0.2 to 10% by weight of maleicanhydride and up to 40% and preferably 5 to 40% by weight of alkyl(meth)acrylate. Their MFIs are between 2 and 100 g/10 min. (190° C./2.16kg). The term “alkyl (meth)acrylate” advantageously denotes C₁ to C₈alkyl acrylates and methacrylates and may be chosen from methylacrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate,2-ethylhexyl acrylate, cyclohexyl acrylate, methyl methacrylate andethyl methacrylate.

[0123] The melting point is between 80 and 120° C. These copolymers arecommercially available. They are produced by radical polymerization at apressure that may be between 200 and 2500 bar.

[0124] By way of example, it is also possible to use the followingblends (in % by weight):

[0125] 1)

[0126] 55 to 70% of PA-6,

[0127] 5 to 15% of an ethylene/propylene copolymer containingpredominantly propylene, grafted with maleic anhydride and thencondensed with monoaminated caprolactam oligomers,

[0128] the balance to 100% of polypropylene;

[0129] 2)

[0130] 55 to 70% of PA-6,

[0131] 5 to 15% of at least one copolymer of ethylene with (i) an alkyl(meth)acrylate or a vinyl ester of an unsaturated carboxylic acid and(ii) an unsaturated carboxylic acid anhydride or an unsaturated epoxide,which is grated or copolymerized,

[0132] the balance of polyethylene;

[0133] 3)

[0134] 55 to 70% of PA-6,

[0135] 5 to 15% of polyethylene or copolymers of ethylene with analpha-olefin, grafted with maleic anhydride or glycidyl methacrylate,

[0136] the balance of high-density polyethylene.

[0137] With regard to the layer (2 a) formed from EVOH copolymer, thismay consist of EVOH or of an EVOH-based blend. EVOH is also referred toas a saponified ethylene/vinyl acetate copolymer. The saponifiedethylene/vinyl acetate copolymer to be used according to the presentinvention is a copolymer having an ethylene content of 20 to 70 mol %,preferably 25 to 70 mol %, the degree of saponification of its vinylacetate component not being less than 95 mol %. With an ethylene contentof less than 20 mol %, the barrier properties under high-humidityconditions are not as high as would be desired, whereas an ethylenecontent exceeding 70 mol % results in reduced barrier properties. Whenthe degree of saponification or hydrolysis is less than 95 mol %, thebarrier properties are sacrificed.

[0138] The expression “barrier properties” is understood to mean theimpermeability to gases and liquids, and in particular to oxygen and topetrol for motor vehicles.

[0139] Among these saponified copolymers, those which have melt flowindices within the 0.5 to 100 g/10 min. range are particularly useful.Advantageously, the MFI is chosen between 5 and 30 g/10 min. (at 230°C./2.16 kg), “MFI” is the abbreviation for “Melt Flow Index”.

[0140] It is known that this saponified copolymer may contain smallamounts of other comonomer ingredients, including α-olefins, such aspropylene, isobutene, α-octene, α-dodecene, α-octadecene, etc.,unsaturated carboxylic acids or their salts, partial alkyl esters,complete alkyl esters, nitriles, amides and anhydrides of the saidacids, and unsaturated sulphonic acids and salts thereof.

[0141] As regards the EVOH-based blends, these are such that the EVOHforms the matrix, that is to say it represents at least 40% andpreferably at least 50% by weight of the blend. The other constituentsof the blend are chosen from polyolefins, polyamides and possiblyfunctional polymers.

[0142] As a first example of these EVOH-based blends, mention may bemade of the following compositions (by weight):

[0143] 55 to 99.5 parts of EVOH copolymer;

[0144] 0.5 to 45 parts of polypropylene and compatibilizer, theirproportions being such that the ratio of the amount of polypropylene tothe amount of compatibilizer is between 1 and 5.

[0145] Advantageously, the ratio of the MFI of the EVOH to the MFI ofthe polypropylene is greater than 5 and preferably between 5 and 25.Advantageously, the MFI of the polypropylene is between 0.5 and 3 (ing/10 min. at 230° C./2.16 kg). According to an advantageous embodiment,the compatibilizer is a polyethylene carrying grafted polyamide speciesand it results from the reaction of (i) a copolymer of ethylene and agrafted or copolymerized unsaturated monomer X with (ii) a polyamide.The copolymer of ethylene and a grafted or copolymerized unsaturatedmonomer X is such that X is copolymerized and it may be chosen fromethylene/maleic anhydride copolymers and ethylene/alkyl(meth)acrylate/maleic anhydride copolymers, these copolymers comprisingfrom 0.2 to 10% by weight maleic anhydride and from 0 to 40% by weightalkyl (meth)acrylate. According to another advantageous embodiment, thecompatibilizer is a polypropylene carrying grafted polyamide specieswhich result from the reaction of (i) a propylene homopolymer or apropylene copolymer comprising a grafted or copolymerized, unsaturatedmonomer X with (ii) a polyamide. Advantageously, X is grafted. Themonomer X is advantageously an unsaturated carboxylic acid anhydridesuch as, for example, maleic anhydride.

[0146] As a second example of these EVOH-based blends, mention may bemade of the compositions comprising:

[0147] 50 to 98% by weight of an EVOH copolymer;

[0148] 1 to 50% by weight of a polyethylene;

[0149] 1 to 15% by weight of a compatibilizer consisting of a blend ofan LLDPE or metallocene polyethylene and-a polymer chosen fromelastomers, very low-density polyethylenes and metallocenepolyethylenes, the blend being cografted by an unsaturated carboxylicacid or a functional derivative of this acid.

[0150] Advantageously, the compatibilizer is such that the MFI₁₀/MFI₂ratio is between 5 and 20, where MFI₂ is the melt flow index at 190° C.with a load of 2.16 kg, measured according to ASTM D1238, and MFI₁₀ isthe melt flow index at 190° C. with a load of 10 kg according to ASTMD1238.

[0151] As a third example of these EVOH-based blends, mention may bemade of the compositions comprising:

[0152] 50 to 98% by weight of an EVOH copolymer;

[0153] 1 to 50% by weight of an ethylene/alkyl (meth) acrylatecopolymer;

[0154] 1 to 15% by weight of a compatibilizer resulting from thereaction of (i) a copolymer of ethylene and a grafted or copolymerizedunsaturated monomer X with (ii) a copolyamide.

[0155] Advantageously, the copolymer of ethylene and a grafted orcopolymerized unsaturated monomer X is such that X is copolymerized, andit is an ethylene/maleic anhydride copolymer or an ethylene/alkyl(meth)acrylate/maleic anhydride copolymer. Advantageously, thesecopolymers comprise from 0.2 to 10% by weight of maleic anhydride andfrom 0 to 40% by weight of alkyl (meth)acrylate.

[0156] With regard to the layer (3) made of a polyamide P3,<<polyamide>> is understood to mean within the context of the presentinvention polyamides or PAs which contain aliphatic and/orcycloaliphatic and/or aromatic units.

[0157] Advantageously, nylon-11 or nylon-12 is used. Advantageously, thepolyamide of the outer layer is plasticized by standard plasticizerssuch as n-butyl benzene sulphonamide (BBSA) and polymers comprisingpolyamide blocks and polyether blocks. These block polymers result fromthe condensation of polyamide blocks having carboxylic end groups witheither polyetherdiols or polyetherdiamines, or a blend of thesepolyethers. This outer layer may also contain antioxidants and standardfillers such as carbon black. In general, the plasticizers of thepolyamide-block and polyether-block type, which may be added to theouter layer, are those described in Patent Application FR 94/14521.

[0158] With regard to the layer (2) formed from a blend of PA-6/12copolyamides, one comprising by weight more 6 than 12 and the other more12 than 6, the PA-6/12 copolyamide results from the condensation ofcaprolactam with lauryllactam. It is clear that “6” denotes the unitsderived from caprolactam and “12” denotes the units derived fromlauryllactam. It would not be outside the scope of the invention ifcaprolactam were to be replaced entirely or partly with aminocaproicacid, and likewise lauryllactam could be replaced with aminododecanoicacid. These copolyamides may include other units, provided that theratio of the 6 and 12 proportions are respected.

[0159] Advantageously, the copolyamide containing more 6 comprises 52 to90% by weight of 6 for 48 to 10% of 12, respectively.

[0160] Preferably, the copolyamide containing more 6 comprises 55 to 90%by weight of 6 for 45 to 10% of 12, respectively.

[0161] More preferably, the copolyamide containing more 6 comprises 55to 70% by weight of 6 for 45 to 30% of 12, respectively.

[0162] Most preferably, the copolyamide containing more 6 comprises 60to 90% by weight of 6 for 40 to 10% of 12, respectively.

[0163] Advantageously, the copolyamide containing more 12 comprises 52to 90% by weight of 12 for 48 to 10% of 6, respectively.

[0164] Preferably, the copolyamide containing more 12 comprises 55 to90% by weight of 12 for 45 to 10% of 6, respectively.

[0165] More preferably, the copolyamide containing more 12 comprises 55to 70% by weight of 12 for 45 to 30% of 6, respectively.

[0166] More preferably, the copolyamide containing more 12 comprises 60to 90% by weight of 12 for 40 to 10% of 6, respectively.

[0167] As regards the proportions of the copolyamide rich in 6 and ofthe copolyamide rich in 12, these may be, by weight, from 40/60 to 60/40and preferably 50/50.

[0168] These copolyamide blends may also include up to 30 parts byweight of other grafted polyolefins or (co)polyamides per 100 parts ofcopolyamides rich in 6 and rich in 12.

[0169] These copolyamides have a melting point (DIN 53736B standard) ofbetween 60 and 200° C. and their relative solution viscosity may bebetween 1.3 and 2.2 (DIN 53727 standard; m-cresol solvent, 0.5 g/100 mlconcentration, 25° C. temperature, Ubbelohde viscometer). Their meltrheology is preferably similar to that of the materials of the adjacentlayers. These products are manufactured by the standard techniques forpolyamides. Processes are described in patents U.S. Pat. No. 4,424,864,U.S. Pat. No. 4,483,975, U.S. Pat. No. 4,774,139, U.S. Pat. No.5,459,230, U.S. Pat. No. 5,489,667, U.S. Pat. No. 5,750,232 and U.S.Pat. No. 5,254,641.

[0170] The invention also relates to tubes consisting of thesestructures; the layer (1) or (1 a) is placed on the inside of the tube.The tubes of the invention may be produced by coextrusion.

[0171] These tubes, intended to take petrol from the tank to the engineof motor vehicles, have an external diameter generally ranging from 6-to12 mm and their thickness varies in general from 0.8 to 2 mm. The layer1 has a thickness in general of at least 50 μm and preferably 100 to 500μm.

[0172] The layer 1 a filled with electrically conducting carbon blackhas a thickness in general ranging from 25 to 300 μm and preferably from50 to 150 μm.

[0173] The layer 2 of the copolyamide blend has a thickness in generalof at least 10 μm and preferably 20 to 100 μm.

[0174] The EVOH layer 2 a has a thickness of 10 to 200 μm.

[0175] The layer 3 has a thickness in general of at least 300 μm andpreferably 400 to 800 μm.

[0176] These multilayer tubes may be cylindrical, with a constantdiameter, or corrugated.

[0177] Conventionally, these tubes may include protective sheaths,especially made of rubber, in order to protect them from engine hotspots.

[0178] The entire disclosure's of all applications, patents andpublications, cited herein and of corresponding French application No.02.01039, filed Jan. 29, 2002, and U.S. Provisional Application SerialNo. 60/358,388, filed Feb. 22, 2002, are incorporated by referenceherein.

1. A polyamide-based multilayer structure comprising: a first layer (1)comprising a polyamide P₁ or a blend of a polyamide P₁ and a polyolefinPO₁ having a P₁ polyamide matrix, optionally, a layer (2 a) comprisingEVOH; a layer (2) comprising a blend of a first PA-6/12 copolyamide,containing more PA-6 by weight than PA-12 and a second PA-6/12copolyamide containing more PA-12 by weight than PA-6; a layer (3)comprising from a polyamide P₃, P₁ and P₃ being identical or different,layers (1), (2), (2 a) and (3) being successive and adhering to oneanother in their respective contact regions.
 2. The structure accordingto claim 1, in which the first layer (1) is a layer (1 a), comprising apolyamide P₁a or a blend of a polyamide P₁a and a polyolefin PO₁a havinga polyamide matrix and containing electrically conducting carbon blackproducing a surface resistivity of less than 10⁶ Ω/□.
 3. The structureaccording to claim 1, comprising an additional layer (1 a) placed on thelayer (1) side, layers (1) and (1 a) adhering to each other in theirrespective contact region, layer (1 a) comprising a polyamide P₁a or ablend of a polyamide P₁a and a polyolefin PO₁a having a polyamide matrixand containing electrically conducting carbon black producing a surfaceresistivity of less than 10⁶ Ω/□.
 4. The structure according to claim 1,in which the polyamide P₁ is nylon-6, nylon-6,6 or nylon-12.
 5. Thestructure according to claim 2, in which the polyamide P₁a is nylon-6,nylon-6,6 or nylon-12.
 6. The structure according to claim 3, in whichthe polyamide P₁ or P₁a is nylon-6, nylon-6,6 or nylon-12.
 7. Thestructure according to claim 1 to 4, in which the polyolefin PO₁ or PO₁ais: polyethylene; polypropylene; an ethylene/alpha-olefin copolymer; anethylene/alkyl (meth)acrylate copolymer; an ethylene/alkyl(meth)acrylate/maleic anhydride copolymer, the maleic anhydride beinggrafted or copolymerized; or an ethylene/alkyl (meth)acrylate/glycidylmethacrylate copolymer, the glycidyl methacrylate being grafted orcopolymerized.
 8. The structure according to claim 1, in which thepolyamide P₃ is a PA-11 or PA-12 polyamide.
 9. The structure accordingto claim 1, in which the blend containing more PA-6 in layer (2)comprises 52 to 90% by weight of PA-6 for 48 to 10% of PA-12,respectively.
 10. The structure according to claim 1, in which the blendcontaining more PA-6 in layer (2) comprises 60 to 90% by weight of PA-6for 40 to 10% of PA-12, respectively.
 11. The structure according toclaim 1, in which the blend containing more PA-12 in layer (2) comprises52 to 90% by weight of PA-12 for 40 to 10% of PA-6, respectively. 12.The structure according to claim 1, in which the blend containing morePA-12 in layer (2) comprises 60 to 90% by weight of PA-12 for 40 to 10%of PA-6, respectively.
 13. The structure according to claim 1, having aproportion of the blend containing PA-6 and of the blend containing morePA-12 in layer (2) of 40/60 to 60/40 by weight.
 14. A tube containing astructure according to claim 1, having an inside layer (1) or (1 a) andan outside layer (3).
 15. A polyamide blend comprising a compatibilizerwhich is a blend of a first PA-6/12 copolyamide, having more PA-6 byweight than PA-12 and a second copolyamide having more PA-12 by weightthan PA-6.
 16. The polyamide blend according to claim 15, which is PA-12and at least one of PA-6 or PA-6,6.
 17. The polyamide blend according toclaim 15, wherein in the compatibilizer the first copolyamide comprises52 to 90% by weight of PA-6 for 40 to 10% of PA-12 respectively.
 18. Thepolyamide blend according to claim 15, wherein in the compatibilizer thesecond copolyamide comprises 52 to 90% by weight of PA-12 for 40 to 10%of PA-6 respectively.
 19. The polyamide blend according to claim 15,wherein in the compatibilizer the first copolyamide comprises 60 to 90%by weight of PA-6 for 40 to 10% of PA-12 respectively.
 20. The polyamideblend according to claim 15, wherein in the compatibilizer the secondcopolyamide comprises 60 to 90% by weight of PA-12 for 40 to 10% of PA-6respectively.